Transmission of immunity to GLV from mother to offspring: Having found that immunity to GLV is transmitted to offspring, it is of importance to analyze the routes of transmission and to determine the carrier of this immunity. Experiments performed during the past year, based mainly on foster nursing have indicated that the nature of immunity is humoral and that the transmission to offspring takes place through both placental blood and milk. The identification of serum and milk components responsible for the neutralization of GLV is planned for the coming year. Repression and derepression of GLV antigens in vivo and in vitro: A system by which the antigenic expression of GLV is alternately lost by transplantation of virus-carrying cells in vivo and recovered by their explantation in vitro has been revealed by our recent work. We intend to study each part of this system separately. First to follow the disappearance of gs1 antigens by transplantation of GLV-carrying tissue culture cells in rats treated with whole body x-irradiation or anti-lymphocytic serum. Six parameters for the expression of GLV will be concomitantly followed: cytoplasmic immunofluorescence, after staining with anti-MuLV-serum, immunodiffusion in agar gel, serum cytotoxicity, electron microscopic examination and inoculation of newborn rats with cell-free-filtrates of tumor tissues. The same parameters will be investigated to follow the explantation in vitro of transplantable tumors.